* Mark Spezzano
Can somebody please explain exactly how the monad functions "sequence" and "sequence_" are meant to work?
The others in this thread have already explained how these functions work, so I'll just give an example how they are used. Consider the following task: to read 30 lines from standard input. For one line you would use an action getLine :: IO String How to execute this 30 times? Haskell has a function replicate :: Int -> a -> [a] which takes a number and produces a list with that number of identical elements. So this is close to what we need: replicate 30 getLine :: [IO String] This is a list containing 30 'getLine' actions. But the list of actions _is not_ an action itself. This is what sequence does -- it transforms a list of actions into a single action which gathers the results into one list. As its name suggests, it does sequencing of actions. sequence $ replicate 30 getLine :: IO [String] Exactly what we need, an action producing a list of lines read. Now, let's consider this code: sequence [ putStrLn $ show i ++ " green bottles standing on the wall" | i <- reverse [1..10] ] :: IO [()] This action prints 10 lines and also returns us gathered results, i.e. 10 '()', one from each putStrLn (recall that putStrLn has type "String -> IO ()". Most probably we don't care about those '()', but they still occupy memory and introduce a space leak as explained here[1]. That's why a version of sequence is introduced which ignores the results of actions and simply returns (). It is called "sequence_". sequence_ :: (Monad m) => [m a] -> m () As a side note, we can rewrite our last example without using list comprehension in the following way: let p i = putStrLn $ show i ++ " green bottles standing on the wall" in sequence_ $ map p $ reverse [1..10] The combinations of sequence and sequence_ with map are so common that they have special names: mapM = \f -> sequence . map f :: (Monad m) => (a -> m b) -> [a] -> m [b] mapM_ = \f -> sequence_ . map f :: (Monad m) => (a -> m b) -> [a] -> m () [1] http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.ht... -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain